In general, biosensors are distinguished from chemical sensors in that they use a sensor layer of biological origin (for example an antibody, cell or enzyme) immobilized on a surface as the target-sensitive component of the sensor. Recent literature by Hunt et al., “Time dependent signatures of acoustic wave biosensors,” IEEE Proceedings, Vol. 91, no. 6 pp. 890-901, June 2003 and Stubbs, D. D., Lee, S. H. and Hunt, W. D., “Investigation of cocaine plumes using surface acoustic wave immmunosassay sensors,” Analytical Chemistry, vol. 75, no. 22, pp. 6231-6235, Nov. 15, 2003, has demonstrated that an acoustic wave biosensor with an immobilized biolayer need not be restricted to the detection of biomolecules within the liquid phase, but can detect low vapour pressure chemical molecules such as bacteria, cocaine and explosives.
The detection of such chemical or biological molecules has in the past involved the use of frequency offset versus time graphs. A simple oscillator circuit having an acoustic wave biosensor within its feedback path has its operating frequency altered in a manner depending on the binding events which take place between the antibody and the chemical or biological substance. The binding events change the mass loading and stiffness parameters of a piezoelectric crystal in the oscillator circuit, which subsequently alters the velocity of the propagating acoustic wave and therefore affects the oscillator feedback element. As a result, the frequency of the oscillator is changed.
An object of this invention is to reduce the mathematical complexity required with previous methods of differentiating and identifying analogous chemical or biological substances with surface acoustic wave biosensors.